This application relates to and incorporates by reference Japanese patent application No. 2001-228246, which was filed on Jul. 27, 2001.
The present invention relates to a flow sensor, and more particularly, to a flow sensor having a diaphragm made of component layers.
A flow sensor shown in Japanese unexamined patent publication Hei 11-194043 includes a lower film having a low level of tensile stress and an upper film that also has a low level of tensile stress. In other words, the upper film (above the device) and the lower film (below the device) have almost the same levels of stress.
In this previous invention, the sets of films above and below the device have low levels of tensile stress, because the higher the tensile stress in the entire film, the lower the film""s resistance to pressure. Compressive stress in the film, on the other hand, makes the film buckle. Therefore, the entire film has a weak tensile stress to maximize the film""s resistance to pressure, while ensuring that the film does not buckle. The long-term stability of the sensor is ensured with low levels of tensile stress in both the upper film and the lower film of the device.
In general, the upper and lower films in such a device consist of combinations of silicon nitride films and silicon oxide films. Silicon nitride films tend to have tensile stress, while silicon oxide films tend to have compressive stress. It is possible to adjust the stress in the upper and lower films of the device to desired levels by adjusting the relative thicknesses of the silicon nitride films and the silicon oxide films in order to achieve low levels of tensile stress. In order to ensure low levels of tensile stress in both the upper and the lower films of the device, each of the upper film and the lower film of the device must include both an oxide film and a nitride film, and the thickness of the oxide film and the thickness of the nitride film must be equal. In other words, the following equation 1 would apply:
(Total thickness of the nitride film in the upper film)/(Total thickness of the oxide film in the upper film)=(Total thickness of the nitride film in the lower film)/(Total thickness of the oxide film in the lower film)
Flow sensors for measuring high flow rates are susceptible to a problem of the air flow separating from the diaphragm (films). In general, the air flow tends to separate when the film is deformed downward in a concave shape. The higher the air flow rate, the more likely this problem is to occur. Flow measurement is difficult beyond the point at which the separation occurs, where there tends to be an irregular vortex.
While a silicon nitride film tends to have a tensile stress, a silicon oxide film tends to have a compressive stress, and a platinum film in the device tends to have a tensile stress. When the sensor is designed according to equation 1, the resulting film tends to have a downward concavity. At high flow rates, the air flow tends to separate from the film, and accurate flow rate measurement becomes difficult.
The present invention addresses this issue by providing a flow sensor capable of highly precise flow rate measurement by preventing the separation of the air flow from the diaphragm.
To achieve this objective, the invention is a flow sensor, which includes upper insulating film layers and lower insulating film layers, and the average stress in the upper insulating film layers is more compressive than the average stress in the lower insulating film layers. The flow sensor includes a diaphragm, and the diaphragm is made of layers that include the insulating film layers. By making the average stress in the upper insulating film layers more compressive than that of the lower insulating film layers, the diaphragm will either remain flat or will deform in the upward direction, which permits accurate flow rate measurements at high flow rates.